Improve atomic_swap_global() to take advantage of associativity.

We now do a single atomic hardware swap and then effectively do 
swaps between the running program instances such that the result
is the same as if they had happened to run a particular ordering
of hardware swaps themselves.

Also cleaned up __atomic_swap_uniform_* built-in implementations
to not take the mask, which they weren't using anyway.

Finishes Issue #56.

tests/atomics-swap.ispc

@@ -0,0 +1,17 @@
+
+export uniform int width() { return programCount; }
+
+uniform int32 s = 1234;
+
+export void f_f(uniform float RET[], uniform float aFOO[]) {
+    float a = aFOO[programIndex]; 
+    float b = 0;
+    if (programIndex & 1) {
+        b = atomic_swap_global(&s, programIndex);
+    }
+    RET[programIndex] = reduce_add(b) + s;
+}
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 1234 + reduce_add(programIndex & 1 ? programIndex : 0);
+}